Mentioning GitHub Issue Numbers (#1583 & #1823)

Our contributing guidelines and pull request template now specify that
commit messages should include all appropriate GitHub issue numbers in
them for the sake of traceability.

CONTRIBUTING.md

@@ -46,7 +46,7 @@ Create a local branch off of `develop` on which to make your changes:
 
 ### Make Your Changes
 
-Do whatever work is necessary to address the issue you're tackling, breaking your work into logical, compilable commits.  Feel free to commit small chunks early and often in your local repository and then use `git rebase -i` to reorganize your commits before sharing.
+Do whatever work is necessary to address the issue you're tackling, breaking your work into logical, compilable commits.  Feel free to commit small chunks early and often in your local repository and then use `git rebase -i` to reorganize your commits before sharing.  Make sure the commit messages you will be sharing reference the appropriate GitHub issue numbers.
 
 ### Update Your Branch
 

PULL_REQUEST_TEMPLATE.md

@@ -64,6 +64,7 @@ areas of the code.  Consider including configure, build, and test log files.
 Go over all the following points, and put an `x` in all the boxes that apply.
 If you are unsure about any of these, please ask—we are here to help.
 -->
+- [ ] My commit messages mention the appropriate GitHub issue numbers.
 - [ ] My code follows the code style of the affected package(s).
 - [ ] My change requires a change to the documentation.
 - [ ] I have updated the documentation accordingly.